The invention relates to a method of producing a Fabry-PeΓrot etalon (hereinafter referred to simply as “etalon”), which is a narrow-band frequency filter to be used for a semiconductor laser excitation solid-state laser apparatus, a high resolution spectroscope, a wavelength multiplex communication and the like.
It is very important in practical use to obtain a single longitudinal mode laser with excellent coherence in a space region as well as a frequency (or wavelength) region from a multiple longitudinal mode laser (i.e. a number of oscillation wavelengths), which is generated in a second harmonic solid laser apparatus using a semiconductor laser as a light source. Therefore, it has been conventionally practiced to install a narrow band frequency filter with high permeability in a laser oscillator to obtain the single longitudinal mode laser with a desired wavelength from a plurality of oscillation spectra.
The etalon is roughly classified into two types, called an air space type etalon and a solid type etalon. The air space type etalon is structured such that a pair of semi-transparent mirrors is fixed by some means with a constant space therebetween. The solid type etalon is structured such that semi-transparent films are coated on both sides of a flat surface plate, respectively. In the air space type etalon, it is required to control the two mirrors to be parallel. On the other hand, since the solid type etalon is formed of one flat surface plate, it is not required to control the parallel position with respect to the other member to obtain a stable performance.
In the solid type etalon, a peak wavelength of transmitted laser is determined by a thickness d of the etalon, a refraction index n of a material of the etalon and an incident angle θ of the laser beam with respect to the etalon. In order to obtain the laser with a desired wavelength (i.e. a wavelength to be extracted by using the etalon from a plurality of the oscillation wavelengths) at a high output efficiency, the theoretical values for these three parameters have to be applied to an optical system with a high precision, so that the peak transmitted wavelength through the etalon matches the desired peak wavelength of the laser. Among the above-stated parameters, the parameter determined by production precision of the etalon is the thickness d.
In order to obtain the laser with desired output efficiency, it is necessary that a displacement between the wavelength extracted by the etalon and the desired wavelength of the laser is less than a full width at a half height of the desired laser peak. To attain this, the thickness of the etalon should be maintained within an allowance of an order of 10 to several nm. However, with the existing technology, the process with such a high precision has been difficult. Therefore, the displacement due to a variation of the thickness d is compensated by the following methods: (i) adjusting an installation angle of the etalon; (ii) changing the refraction index n; (iii) changing the refraction index n and the thickness d by controlling a temperature. However, it is practically impossible to change the refraction index, and a structure of the laser resonator becomes complicated to change the installation angle or control the temperature, resulting in a higher cost.
In view of the above problems, the present invention has been made and an object of the invention is to provide a method of producing the etalon with a desired thickness at high precision.
Further objects and advantages of the invention will be apparent from the following description of the invention.